CANONS blazed and aircraft deployed their payloads, but not a drop of blood was shed in the battle between the cities of Pingdingshan and Zhoukou in Henan province, China, on 10 July last year. That’s because their targets weren’t people, but clouds.

The blistering dry spell that struck the province threatened to cause water shortages and ruin crops. The problem was frustratingly clear: though clouds drifted across the skies, too few would give up their water as rain. To coax the water out, Pingdingshan meteorologists decided to resort to a controversial practice they believed would help the heavens break, called cloud seeding. Anti-aircraft guns and rockets were used to bombard pregnant clouds with a fine spray of silver iodide crystals in the hope they would prompt large droplets to form in the cloud, and thus produce more rain.

A few hours later it looked like their gambit had paid off. Westerly winds blew the clouds over Pingdingshan, where they dropped 10 centimetres of rain. But later that day only 2.5 centimetres of rain fell over Zhoukou, further east.

Meteorologists at Zhoukou cried foul. In the war of words that followed, they claimed Pingdingshan’s cloud seeding had caused rain that would have fallen over Zhoukou to fall early. Nonsense, Pingdingshan officials retorted: clouds are complex systems, and seeding shouldn’t affect the chance of rain 120 kilometres away.

“There’s no scientific evidence that cloud seeding can dry clouds out,” says Yan Yin, an atmospheric physicist at the University of Wales in Aberystwyth, UK. “But I’m not surprised they quarrelled. In China the effectiveness of cloud seeding is an article of faith. They seed clouds all the time.” Indeed, 23 of its 34 provinces have “weather modification bureaux” and routinely seed clouds. But does showering a cloud with chemical particles really cause more rain to fall?

Take a glance at many other countries around the world and you’d be forgiven for thinking the answer is a clear-cut yes. Australia, the United States, Israel, South Africa, Russia and India all have extensive private or public cloud seeding programmes, some of which have been operating for over 50 years (see Map). Governments and businesses spend millions in the hope of wringing moisture from the clouds.

In reality the facts are far from clear. “There’s no good statistical proof that cloud seeding produces more rainfall than not seeding,” says Phil Brown, manager of cloud physics research at the UK’s Met Office in Exeter. And even though meteorologists know how cloud seeding is supposed to work, it is difficult to measure its effectiveness. “You have to prove it was your seeding that produced the rainfall increase rather than some other effect,” says Brown. And that is no mean feat.

Proof that it works will bring great rewards. Fresh water is fast becoming one of the most valuable resources on the planet. That is why scientists all over the world have spent the past 50 years trying to prove cloud seeding works. To date, no experiment has proved conclusive either way. But now a team from South Africa has completed the first long-term study of cloud seeding, and the researchers believe they at last have the evidence that, under certain conditions, cloud seeding really does increase rainfall. They claim that their method can increase the mass of water droplets in a cloud – and therefore the amount of rainfall – by up to 60 per cent (see Graph).

Clouds form in two phases. When there is enough water vapour in the air, water can condense onto naturally occurring hygroscopic – moisture absorbing – particles (such as salt crystals or dust) called cloud condensation nuclei (CCNs), to form microscopic droplets.

If the droplets are lifted by air currents, the falling temperature and pressure increase the relative humidity to a point where they can absorb more water and grow far larger. These can fall as drizzle – 100-micrometre droplets – or rain, with droplets larger than 1 millimetre (see Diagram).

In really cold clouds (below -38 °C) there is an additional stage. As the moist air rises the droplets freeze. These ice crystals can continue to grow if they collide with other ice crystals. When they fall, the air temperature below the cloud determines whether they reach the ground as snow, or melt and fall as rain.

The aim of cloud seeding is to optimise the number of droplets that form in the cloud. This is tricky. Small cloud droplets tend to grow faster than large ones, so you often end up with droplets of similar size, which therefore fall at the same speed and seldom collide – meaning no rain. Only clouds with a variety of droplet sizes will produce rain. So, the key to cloud seeding is introducing the optimum number of CCNs into a cloud – too few and the droplets won’t form, too many and you end up with lots of tiny droplets that don’t coalesce enough to fall out of the sky.

Because of their differing precipitation mechanisms, cold clouds and warm clouds require different seeding methods. Cold cloud seeding was discovered accidentally in 1946 by Vincent Schaefer, a scientist working at the General Electric labs in New York. He observed that spraying crystals of frozen carbon dioxide into a cold cloud chamber helped ice crystals to form. He reasoned that scattering dry ice into cold clouds would have the same effect, helping initiate precipitation. In his first trial, he scattered 1.5 kilograms of crushed dry ice into stratocumulus clouds over western Massachusetts. Because dry ice is cumbersome to carry by plane, he later switched to silver iodide – a chemical with a similar molecular structure to ice crystals – as the seed, hoping it would kick-start crystal formation in a similar way to carbon dioxide. Though the experiments were inconclusive, the idea gained attention and became known as glaciogenic seeding.

Warm cloud seeding didn’t arrive until 1989. A group of South African scientists, led by the late Graeme Mather, were using an aircraft to measure the droplet properties of various clouds as part of a cold cloud seeding study. One day they sampled a very unusual cumulus cloud above a large paper mill. It had exceptionally large droplets – around half a centimetre wide – in its updrafts and a surprisingly wide range of droplet sizes at its base.

Unlike other cumulus clouds they studied, the cloud above the paper mill was producing big drops of rain. Mather and his colleagues worked out that the paper mill was inadvertently seeding the cloud above because it pumped hygroscopic particles that acted as CCNs out of its chimney stack. They were the first to realise that the large particle size was crucial, and that mimicking these hygroscopic particles could be a way to seed warm clouds and produce large raindrops. The most hygroscopic natural CCN material is common salt, so they decided to test warm cloud seeding using flares stuffed with salt crystals strapped to an aircraft wing.

Early trials were encouraging, so in 1991 Mather and his team gave up on glaciogenic seeding and started an extensive long-term hygroscopic cloud seeding trial. They devised a blind, randomised, controlled experiment, and over the course of five summers in the Highveld region of South Africa they sampled 127 warm convective storm clouds – 62 seeded and 65 left as controls. Mather’s team used airborne radar to measure the average density of droplets in the clouds, which they believed would give a good indication of how much rain the cloud would produce. When they compared the data from seeded and unseeded storm clouds, they found seeding really was having an effect. “Seeded clouds evolved to have a longer duration, larger area and higher liquid content,” says André Görgens, a water resources engineer on the team. On average the total mass of water droplets formed in one hour inside a seeded cloud was 60 per cent larger than for an unseeded cloud.

As a result the South African government funded the team to carry out experimental seeding programmes in the drought-stricken Limpopo province in 1997. The aim was to seed as many suitable storm clouds as possible with full scientific monitoring, and then carry out a cost-benefit analysis and environmental assessment. Over the past four years Görgens and his colleagues have been analysing the data gathered during the programme and published several papers on their findings (Water South Africa, vol 30, special edition, p 88).

They calculated that seeding just a quarter of the storm clouds in the Limpopo province would increase average rainfall by up to 10 per cent. And Görgens’s cost-benefit analysis concluded that the benefits of cloud seeding outweigh the costs by a factor of 1.7. “Cloud seeding helps to keep the reservoir levels up and enables an increase in rain-fed agricultural output, such as maize, grazing and timber production,” he says.

Yin is impressed with Görgens’s study. “These results are based on a statistically sound experiment and can be trusted,” he says. And that’s rare for cloud seeding experiments.

It’s rain that matters

But there is one uncomfortable hole in Görgens’s trial, as Peter Hobbs, an atmospheric physicist at the University of Washington in Seattle, points out. It’s not enough to measure the density of drops in a cloud, he says. Where is the proof that the raindrops hit the ground? “Seeding can change the structure of clouds, but it is a different issue as to whether it increases rainfall.” Hobbs thinks the only way to be sure is to design an experiment with a dense network of gauges to measure rainfall on the ground.

Görgens agrees that such measurements would be desirable, but says he has anecdotal evidence from local farmers that rainfall increased during storms he seeded. In future tests he hopes they will get a more accurate impression of rainfall thanks to a network of weather radars that is currently being set up across South Africa.

And of course, even if cloud seeding does work, it can only squeeze rain from existing clouds, not create new ones. “Most areas that are short of rain are also short of clouds, and you can’t seed if there aren’t any clouds,” says Hobbs.

On the other hand, one of the major advantages of hygroscopic seeding is that warm cumulus clouds do tend to pop up in arid areas, unlike the large cold clouds required for glaciogenic seeding. Roelof Bruintjes, now at the National Center for Atmospheric Research in Colorado, and originally a member of Mather’s South African cloud-seeding team, has exported the hygroscopic seeding method to other arid areas in the world.

Between 1996 and 1999 he reproduced Mather’s South African experiments in Mexico and recorded increases in droplet density of up to 30 per cent. Last year he finished a feasibility study of hygroscopic seeding in the deserts of the United Arab Emirates. Clouds are rare in the UAE, but Bruintjes reckons he has found some suitable candidates. “During the summer months thunderstorms build over the Oman Mountains, and these are feasible clouds for hygroscopic seeding,” he says. He is now analysing the results, and if they are positive, he expects to start a full seeding programme in the near future.

Glaciogenic seeding has had a more chequered past. To date, most studies appear to fall short in scientific rigour. Hobbs is dismissive of studies from Israel and Colorado. “When I re-analysed their results I found that the experiments hadn’t been randomised properly and they didn’t have enough trials to reach statistical significance,” he says.

All this could be about to change. Up in the Snowy Mountains in New South Wales, Australia, hydroelectric power company Snowy Hydro is one year into a six-year trial to see if glaciogenic cloud seeding will make winter snows heavier. Rather than spray silver iodide from aeroplanes, they are using ground-based generators that spray it directly into clouds as they rise over the mountainside. Just like the South Africa experiments, they have designed their trials to be double blind. They will monitor the clouds with radar and use a dense network of snow gauges to measure any change in snowfall at the surface.

But could cloud seeding be dangerous? Some people worry that seeding a heavily laden cloud could cause flash floods (see “Murky past of cloud seeding”). “It is not beyond the bounds of possibility,” says Hobbs, “but I personally doubt it.” Görgens thinks that choosing your clouds carefully will eliminate the risk of floods. However, he admits the extra water could attract more insects. Others are concerned about silver iodide pollution. But Mark Heggli of Snowy Hydro says his ground measurements after seeding have barely detected even trace levels – too low to be a danger to human health. “You get a bigger dose of silver from fillings in your teeth,” he says.

Perhaps the greatest worry is the potential for cloud seeding to be used as a weapon. In 1977 the UN General Assembly adopted a resolution prohibiting the hostile use of environmental modification techniques. Yet nine years ago the US air force commissioned a report entitled Weather as a Force Multiplier: Owning the weather in 2025. The report concludes, “Over the course of the next century, the weather will be our most powerful weapon. Weather modification can provide battlespace dominance to a degree never before imagined. By 2025 it will be in the realm of possibility.”

Görgens prefers to focus on the benefits of cloud seeding. He cites indigenous tribes such as the Pedi, who live in the bushveld of the Limpopo province, as potential beneficiaries. “These people live by subsistence farming and they are very vulnerable to a shortfall in the rain. Warm cloud seeding would bring more reliable rains for them.”

When Snowy Hydro’s experiment is concluded we should also have a clearer picture of the effects of glaciogenic seeding, by far the most popular method around the world at the moment. But even if the results are positive, it won’t clear up the controversy around cloud seeding. Just ask the people of Zhoukou.

Murky past of cloud seeding

On 15 August 1952, the people of Lynmouth in south-west England experienced a downpour like never before. The ensuing floods killed 34 people and left 420 more homeless. At the time rumours circulated that the flood could have been the result of cloud seeding trials that the Royal Air Force was conducting nearby. However, few people now think that the cloud seeding was to blame, because the RAF was seeding cumulus clouds, while the rain that deluged Lynmouth came from a large depression sitting over the region.

In 1966 the US military began operational flights on the CIA-inspired, top secret “Project Popeye”. Their aim was to extend the monsoon season over south-east Asia, thereby increasing the amount of mud on the Ho Chi Minh Trail and flooding critical routes between what were then North Vietnam and South Vietnam. For seven years, aircraft flew more than 2600 sorties to disperse silver iodide into the clouds over the region, and initial results were positive, although analysts remain divided as to whether the extra mud on the trail really made much difference.

On 9 June 1972, 238 people lost their lives in floods in Rapid City, South Dakota, when nearly a year’s worth of rain fell in just a few hours. Cloud seeding had been carried out nearby earlier in the day and many people believed this triggered the storm. But Harold Orville, who was involved with the seeding programme, is certain that it was not responsible. “Our seeding was done on the plains, 20 or 30 miles from the area where the heavy rains fell. Numerical simulations of the weather that day suggest that the flood would have happened anyway,” he says.